The invention relates to an apparatus and method for supplying and controlling electrical power to a primary load and to a secondary load.
Power sources used for supplying electrical power to a primary load generally have significant internal impedance, so that as the load increases the internal impedance has an increasingly significant effect on the output voltage of the source. Therefore, the voltage seen by a load driven by a voltage source will droop as he load draws more current because of the increase in voltage dropped across the source impedance. Where a much smaller secondary load is also driven by the power source, the voltage droop may have a disproportionate effect on the voltage range experienced at the input to the secondary load. If the same source supplies a plurality of primary loads which may draw power at independently varying times, the change in the range of voltage suppliable to the secondary load may be greater still.
A particular problem has been experienced in the provision of a regulated power supply to provide stable current and voltage to the secondary load, as is desirable, for example, where the secondary load includes electronic circuitry. This is because regulation of such a power supply becomes more complex and expensive the wider the range of input voltage it has to cope with.
The invention provides an apparatus for supplying and controlling electrical power, comprising:
a power source for supplying electrical power to a primary load and a secondary load;
a capacitor connected to an input to the secondary load; and
control means operable to cause an operating condition of the apparatus to temporarily change from;
a primary operating condition in which power from the power source is supplied to the primary and secondary loads to;
an alternative operating condition in which power from the power source is not supplied to the primary load but is supplied to the secondary load;
thereby increasing the minimum voltage the capacitor can maintain at the input to the secondary load.
The invention is particularly beneficial when used in association with a high power primary load, for example an electric motor driving a mechanical actuator, and a relatively low power secondary load, for example electronic control circuitry. The arrangement is particularly advantageous if the apparatus is in the alternative operating condition for relatively short time periods separated by relatively large time intervals because the effect of such short interruptions in power to the primary load should have little or negligible effect in practice on the performance of the primary load.
The invention is advantageous in that the voltage seen by the secondary load when the primary load no longer draws current from the power source may approach or, in some embodiments, exceed open circuit voltage.
Preferably, the capacitor is connected to the input of a power supply device for supplying electronic circuitry.
The range of voltage seen by the input to the power supply device can thus be reduced, thereby simplifying regulation of the power supply device. This facilitates the provision of a stable power supply, for example for supplying electronic circuitry, without the necessity for complex regulation arrangements which inevitably involve consequential increased manufacturing costs and/or increased weight and/or reduced reliability.
The apparatus may be operable to change between the primary and the alternative operating conditions in response to an instant operating parameter, for example a voltage at the input to the, secondary load. This facilitates more accurate control of the voltage seen by the input to the secondary load, and is also more energy efficient because the power supplied to the primary load is only interrupted when necessary.
Conveniently, the apparatus comprises a comparator operable to repeatedly compare an operating value representing the voltage at the input to the secondary load with a predetermined reference value and to communicate to the control means the result of the comparisons for enabling the control means to temporarily change from the primary operating condition to the alternative operating condition when the voltage at the input to the secondary load falls below a minimum desire level. In this manner the apparatus can be arranged to control the voltage at the input to the secondary load to be above a minimum desired level.
Alternatively, the control means may be operable to temporarily change the operating condition of the apparatus from the primary to the alternative operating condition for predetermined time periods and/or at predetermined time intervals. This facilitates reduction of the complexity and/or weight and/or manufacturing cost of the apparatus.
Preferably, the control means, in the alternative operating condition of the apparatus, is operable to disconnect the primary load from the power source and provide a short circuit across the primary load. This facilitates the provision of relatively simple control circuitry.
Alternatively, the control means is operable to reverse the flow of current from the power source to the load in the alternative operating condition of the apparatus. This enables power from the primary load to be used to regenerate the power source during the selected time periods. For example, where the primary load comprises an ac motor, collapse of flux in the motor windings when power from the source to the motor is switched off can be used to create a reverse current through the source at a higher voltage than the open circuit voltage of the source.
Preferably, a device is provided for preventing current flowing from the secondary load input to the power source. This device is conveniently a diode. This enables the voltage at the input to the secondary load to be greater than the output voltage of the power source under certain operating conditions.
The power source is preferably a d.c. power source, and may comprise a fuel cell or a battery of fuel cells.
The control means preferably includes a d.c. to a.c. converter for supplying a primary load comprising an a.c. motor. The converter conveniently comprises switch means operable to switch the apparatus from the primary operating condition to the alternative operating condition. The switch means may comprise an electronic circuit including semiconductor switches.
The control means preferably includes a programmable electronic control device.
The power supply device is preferably a regulated d.c. power supply for supplying electronic control circuitry.
The time constant of the capacitor may be of the order of tens of microseconds.
The invention also includes a method of supplying and controlling electrical power from a power source to a primary load and to a secondary load which has a capacitor connected to an input thereof, comprising temporarily interrupting the supply of power from the power source to the primary load, thereby increasing the minimum voltage the capacitor can maintain at the input to the secondary load.
The time periods of the interruptions and/or intervals between the interruptions may be selected in response to an instant operating parameter, for example a voltage measured at the input to the secondary load.
Alternatively, the time periods of the interruptions and/or intervals between the interruptions may be predetermined.
Preferably, the time periods are short compared to the time intervals. The time periods are conveniently in the range 10 to 20 microseconds. On the other hand, the time intervals are conveniently of the order of 2 milliseconds.
The supply of power from the power source to the primary load may be interrupted and the load short circuited. Alternatively the supply of power from the power source to the primary load may be interrupted and the current to the primary load reversed.
Using the latter method, the interruption of power to the primary load is conveniently used to cause a collapse in flux in the windings of an a.c. motor, the resulting motor generated voltage being applied to the power source and seen by the input to the secondary load. Such a motor generated voltage may be higher than the open circuit voltage of the power source. This facilitates recharging of the capacitor at a higher rate, and/or regeneration of the power source.